1. Field of the Invention
This invention relates to apparatus and method for determining the rate of flow by a magnetic flow meter that measures an electrical potential difference developed in the fluid as the fluid moves through a magnetic field.
2. Background Information
In most prior art in-line magnetic flow meters, the electrical potential difference developed in the fluid is sensed by a pair of electrodes contacting the fluid and spaced apart from each other by the diameter of a round flow sensing passage containing the fluid. A magnetic field orthogonal to both the line between the electrodes and the direction of flow through the sensing passage is commonly provided by two coils of wire located on opposite sides of and outside the passage. Sophisticated electronics are used to energize the coils to generate the magnetic field, to amplify the tiny flow-related signals generated, and to reject various noise and drift signal components which would otherwise degrade measurement accuracy. These meters are characterized by an unobstructed flow passage offering very low pressure drop and high tolerance to solids in the fluid, high measurement precision, high power consumption and high cost.
In his U.S. Pat. No. 6,571,642 and in his pending patent application Ser. No. 12/206,881, the disclosures of which are herein incorporated by reference, the inventor taught in-line magnetic flow meters providing an extended flow sensing path between paired sensing electrodes. These meters also have a more efficient magnetic circuit which produces higher flow-generated signals. The extended path was configured as a quasi-annulus formed by partially or completely obstructing the central part of the fluid flow path. Although this arrangement increases the pressure drop caused by the meter, it also provides a higher measurement precision, and lower power consumption due to the longer sensing path and the more efficient magnetic circuit.
The flow responsive voltages detected at the electrodes of prior art magnetic flow meters are typically reduced in magnitude, to at least some extent, by the electrical impedance shunting effect of the fluid on the electrodes. This usually occurs because the voltage generated in the fluid a short distance from the electrodes is generally lower than that sensed by the electrodes. This situation arises because the magnetic field is usually centered on the electrodes and has a strength that falls off with distance away from those electrodes. Straightforward approaches of making the magnetic field more uniform over a larger area generally increase power requirement, coil size, etc. Thus, magnetic flow meters would benefit from an arrangement that reduces shunt loading at the sensing electrodes without exacting a penalty of substantially increased power consumption.